title: Three-dimensional flexibility characteristics of the human cervical spine in vivo.
abstract: METHODS
  A test-retest design to establish the reliability of a new system capable of quantifying the load-displacement characteristics of the cervical spine. The study was primarily descriptive, but the design allowed comparisons between men and women as well as within-group comparisons among different cervical motions.
  
  
  OBJECTIVE
  To determine the flexibility of the entire cervical spine in vivo and to establish the reliability of a new system developed for this purpose.
  
  
  BACKGROUND
  The flexibility of the cervical spine has been studied primarily in vitro by applying loads to isolated osteoligamentous segments. Quantification of the mechanical characteristics of the cervical spine in vivo may provide insights to the effects of pathology and treatment interventions. In vivo flexibility measurements differ from those in vitro in that they involve the entire cervical spine composite, including the muscles, rather than isolated segments.
  
  
  METHODS
  Our method uses a 6 degrees of freedom mechanical linkage system aligned anatomically according to Grood and Suntay parameters and allows manual application of torque around each axis. We determined the range of motion and flexibility of the cervical spine in a sample of young, healthy subjects (n = 20) for flexion, right lateral bending, and bilateral axial rotation.
  
  
  RESULTS
  Acceptable test-retest reliability were found for range of motion and flexibility measurements performed several days apart. The general shape of the torque-angle curves was nonlinear and biphasic. An early, very flexible portion of the curve was defined as the neutral zone, and the less flexible, end portion of the curve was defined as the elastic zone. We found that men were less flexible than women and that men could tolerate greater amounts of passively applied torques. All subjects showed significantly greater flexibility and less torque tolerance in axial rotation compared with those values in flexion and lateral bending. Possible anatomic explanations for these differences include the effect of muscle alignment and flexibility differences between synovial and fibrocartilaginous articulations.
  
  
  CONCLUSIONS
  This study provides data regarding the in vivo flexibility of the human neck in young, healthy subjects and forms the basis for comparison in future studies that assess the effects of pathology and treatment. Men have lower flexibility than women, and axial rotation flexibility is significantly greater than that in lateral bending and flexion.
authors.affiliation: Department of Physical Therapy, Allegheny University of the Health Sciences, Philadelphia, Pennsylvania, USA.
authors.forename: P
authors.initials: P
authors.lastname: McClure
authors.forename: S
authors.initials: S
authors.lastname: Siegler
authors.forename: R
authors.initials: R
authors.lastname: Nobilini
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D008722:Methods; D009333:Neck; D009334:Neck Muscles / Q000502:physiology; D016059:Range of Motion, Articular / Q000502:physiology*; D012016:Reference Values; D012399:Rotation; D012727:Sex Characteristics; D013131:Spine / Q000502:physiology*; D019415:Torque